The present invention relates to ceramic articles, and more particularly to ceramic articles having properties suitable for use in exhaust after-treatment applications, particularly diesel exhaust filtration.
Recently, much interest has been directed towards the diesel engine due to its efficiency, durability and economical aspects. However, diesel emissions have been scrutinized both in the United States and Europe, for their harmful effects on the environment and on humans. As such, stricter environmental regulations will require diesel engines to be held to the same standards as gasoline engines. Therefore, diesel engine manufacturers and emission-control companies are working to achieve a diesel engine which is faster, cleaner and meets the most stringent of requirements under all operating conditions with minimal cost to the consumer.
One of the biggest challenges in lowering diesel emissions is controlling the levels of diesel particulate material present in the diesel exhaust stream. In 1998 diesel particulates were declared a toxic air contaminant by the California Air Resources Board. Legislation has been passed that regulates the concentration and particle size of diesel particulate pollution originating from both mobile and stationary sources.
Diesel particulate material consists mainly of carbon soot. One way of removing the carbon soot from the diesel exhaust is through diesel traps. The most widely used diesel trap is the diesel particulate filter which filters the diesel exhaust by capturing the soot on the porous walls of the filter body. The diesel particulate filter is designed to provide for nearly complete filtration of soot without significantly hindering the exhaust flow. However, as the layer of soot collects on the surfaces of the inlet channels of the diesel particulate filter, the lower permeability of the soot layer causes a gradual rise in the back pressure of the filter against the engine, causing the engine to work harder. Once the carbon in the filter has accumulated to some level, the filter must be regenerated by burning the soot, thereby restoring the back pressure to low levels. Normally, the regeneration is accomplished under controlled conditions of engine management whereby a slow burn is initiated and lasts a number of minutes, during which the temperature in the filter rises from about 400-600° C. to a maximum of about 800-1000° C.
Porous, wall-flow ceramic filters have been utilized for the removal of carbonaceous soot particulates from the exhaust of diesel engines for more than twenty years. Ideally, a porous ceramic diesel particulate filter (DPF) should combine low CTE (for thermal shock resistance), low pressure drop (for engine efficiency, and fuel economy), high filtration efficiency (for removal of most particles from the exhaust stream), high strength (to survive handing, canning, and vibration in use), and low cost. To this end, the primary attributes effecting flow distribution and backpressure are (i) substrate microstructure, (ii) catalyst coating, (iii) soot loadings, (iv) filter geometry and (v) upstream exhaust flow rate and distribution. For a given filter geometry and upstream flow distribution and soot physical properties, the substrate microstructure, the catalystcoating distribution and the soot loadings (deep-in-wall, and soot cake) determine the resulting backpressure of the filter.
Conventional microstructure design of a DPF substrate has been mainly focused on substrate mean porosity (∈) and the median pore size (d50). In fact, two filters with the same porosity and same median pore size (d50) may have different backpressures even with same soot loading and at same flow rate, because of different pore size distributions (PSD) and/or different pore morphologies. The pore size distribution and pore morphology also contribute to the permeability of porous media, which consequently can lead to different filter backpressures even with the same porosity and same median pore size.
The quantitative impact of pore size distribution and pore morphology on the backpressure of DPF's have not been well understood in prior art yet. There is no teaching available in prior art about the control of the full pore size distribution in the way that can lead DPF's that provide for lower backpressures even at relatively high soot loadings (>5 gram/L). In particular, there has been no quantitative relationship available among substrate pore size distribution, pore morphology and porous-media permeability. Most of filter designs in prior art have been focused on the filter geometry optimization (such as cell density, web thickness), and substrate microstructure optimization such as the mean porosity and median pore size (d50). Accordingly, there is a need in the art for a greater understanding of this quantitative relationship, and improved DPF's resulting from same.